Optical amplifier and laser

ABSTRACT

A laser has a resonant cavity defined by a pair of mirrors butted to respective ends of a fluorozirconate optical fiber. The fiber has a numerical aperture of 0.205 and an LP 11  mode cut-off of about 2.0 μm. The fibre is co-doped with thulium ions to a concentration of about 0.1%, and with terbium ions to a concentration of about 1%. An optical pump source provides a pump signal at 775 nm which excites the thulium ions into the  1  G 4  energy level to provide lasing at about 475 nm. The pump source is preferably a high power semiconductor laser.

FIELD OF THE INVENTION

This invention relates to an optical amplifier, and in particular to alaser incorporating an optical amplifier.

BACKGROUND OF THE INVENTION

The impact that an efficient, inexpensive and reliable visible lasersource would have on data storage, display technology, underseacommunications and optical processing has provided the stimulus for muchrecent work on solid state visible lasers. One approach that has yieldedmuch success is the use of upconversion processes within rare earthdoped materials to produce laser emission at a wavelength significantlyshorter than the pump wavelength. For example, there has been a recentdemonstration of visible lasing at 480 nm in Tm³⁺ --doped fluoride fibre(see Allain, J. Y., Monerie, M and Poignant, H.: `Blue upconversionfluorozirconate fibre laser`, Electron. Lett., 1990, 26 (3), pp.166-168), and a demonstration of room temperature lasing at red, greenand blue wavelengths in praseodymium doped fluorozirconate glass fibre(see Smart, R. G., Hanna, D. C., Tropper, A. C., Davey, S. T., Carter,S. F., Szebesta, D,: `CW upconversion lasing at blue, green and redwavelengths in an infrared-pumped Pr³⁺ -doped fluoride fibre at roomtemperature`, Electron. Lett., 1991, 27, (14), pp 1307-1309). Forcedoscillation on two transitions simultaneously has also been demonstrated(see Percival, R. M., Szebesta, D., and Davey, S. T.: "Highly efficientand tunable operation of two colour Tm-doped fluoride fibre laser"Electron. Lett., 1992, 28, (7), pp. 671-672).

These demonstrations have dramatically changed the viability of suchupconversion pumped laser schemes, and recently a significant amount oftime has been spent investigating the infrared emission which emanatesfrom the ³ F₄ manifold in fluoride fibres doped with thulium. During thecourse of this work, it has become common knowledge that, when pumped ataround 790 nm, the fibres glow in the blue region of the electromagneticspectrum. One explanation for this effect, is that a first pump photonresults in population being excited into the ³ F₄ manifold (see FIG. 1which is an energy level diagram of a thulium/terbium co-doped fluoridefibre). From this level, there are three routes for radiative decay,0.805 μm (³ F₄ -³ H₆), 1.475 μm (³ F₄ -³ H₄), and 2.310 μm (³ F₄ -³ H₅)with branching ratios of 0.893 0.083 and 0.024 respectively. The energygap to the next level is sufficiently large that non-radiative decay isprecluded. Thus besides the ground state (³ H₆) and the ³ F₄ manifoldthere will be small populations in the ³ H₄ and ³ H₅ manifolds whenunder excitation. Moreover, the energy gap between the ³ H₅ and ¹ G₄manifolds is quite closely matched to the pump photon energy.Consequently, the sequential absorption of two pump photons could leadto a small fraction of the population excited into the ³ H₅ manifoldreaching a high enough level (¹ G₄) to give rise to a small amount ofblue emission when the excited ion subsequently decays back down to theground state manifold ³ H₆.

The 1992 Electronics Letter paper referred to above observed that theblue emission intensity increased significantly when stimulated emissionwas obtained on the 2.31 μm transition, since the population in the ³ H₅manifold rapidly increased under these circumstances. However, thisscheme is thought to be unworkable as a blue laser, since the fibreparameters for operation at mid-infrared and blue wavelengths are widelydivergent.

SUMMARY OF THE INVENTION

The applicants believe that significant gain at 475 nm can be achievedwhen thulium/terbium co-doped fluorozirconate fibre is pumped at 775 nm.The 475 nm emission observed originates from the ¹ G₄ level and requiresthe sequential absorption of 775 nm pump photons. The present inventionis based on the observation by the applicants that there is bluefluorescence which is ascribed to a transition between the ¹ G₄ leveland the ³ H₆ ground state of the system FIG. 1 represents the energylevels of the thulium and terbium ions, with the relevant blue lasingtransition indicated between the ¹ G₄ level and the ³ H₆ ground state.The upper laser level may be populated by the sequential absorption of775 nm pump photons in a process which involves excitation of groundstate ions into the ³ F₄ band, some of which then branch into the ³ H₅level. These ions are then further excited by pump photons into the ¹ G₄level. A direct transition from this level to the ground state isresponsible for the blue emission.

The present invention is based on an optical amplifier comprising afluorozirconate waveguide co-doped with thulium and terbium ions, and anoptical pump means coupled to the waveguide for providing an opticalpump signal capable of exciting the thulium ions into the ¹ G₄ energylevel, whereby the amplifier provides optical gain at about 475 nm, saidoptical pump means being adopted to provide an optical pump signalhaving a wavelength in the range of 770 nm to 790 nm, preferably about775 nm.

The waveguide may conveniently comprise a fluorozirconate optical fibrewaveguide such as a standard ZBLAN fluorozirconate fibre, but othertypes of waveguide may be employed. For example, it is expected that auseful configuration would be a planar waveguide structure formed bydoping a fluorozirconate glass substrate. High dopant concentrationwould lead to compact (short waveguide length) devices.

The pump, preferably a semiconductor laser diode, may be coupled to thefibre by any known appropriate technique. For example, the highhumerical aperture fluorozirconate fibre may be jointed to a silicafibre so that readily-available fused couplers, for example, can be usedto couple pump and signal sources to the doped fibre.

Advantageously, the fluorozirconate waveguide is doped with thulium ionsto a concentration of about 0.1%, and with terbium ions to aconcentration of about 1%.

The thulium ions are excited into the ¹ G₄ level by sequentialabsorption of two pump photons. The first pump photon is absorbed by athulium ion to excite that ion from the ground state (³ H₆) to the level³ F₄. The excited ions thereafter decay into the ³ H₅ level and thisdecay is assisted by energy transfer to the terbium ions. After decay,the second pump photon is absorbed by the thulium ion to excite that ionto the ¹ G₄ level.

The invention also provides a laser comprising an optical amplifier anda pair of reflectors, the optical amplifier being as defined above, andthe reflectors being positioned one at each end of the waveguide, thereflectors defining a resonant cavity and having reflectivities such asto provide lasing action at about 475 nm when the waveguide is pumped bythe pumping means.

The reflectors, which may be mirrors or other reflectors such as Sagnacloop reflectors, define a Fabry-Perot cavity, and in known manner areselected to provide reflections sufficient to sustain lasing only at thedesired wavelength.

The invention also provides for the amplification of attenuated opticalsignals at about 475 nm. This requires an optical amplifier as describedabove which is provided with an input port for accepting attenuatedsignals at said wavelengths and an output port for providing saidsignals after amplification. For amplification of attenuated signals, itis necessary to minimise, ideally to eliminate, reflections and feedbackof the amplified signal because reflections tend to cause selfsustaining generation of light and this constitutes an unacceptablenoise.

The invention also includes a method of amplifying optical signal whichmethod comprises providing pump radiation at wavelengths in the range770 nm to 790 nm into a fluorozirconate waveguide co-doped with thuliumand terbium ions whereby said pump radiation produces a populationinversion in the terbium ions said population inversion being capable ofamplifying radiation having a wavelength of about 475 nm.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in greater detail, by way ofexample, with reference to the accompanying drawings, in which:

FIG. 1 is a diagram showing the energy levels of thulium and terbiumions in a ZBLAN host;

FIG. 2 is a graph of the intensity against wavelength for blue emissionfrom the ¹ G₄ manifold;

FIG. 3 is a schematic diagram of a first form of laser constructed inaccordance with the invention; and

FIG. 4 is a schematic diagram of a second form of laser constructed inaccordance with the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring to FIG. 1, lasing at a wavelength of 475 nm is achieved byestablishing a population inversion between the ¹ G₄ upper level and the³ H₆ ground state. Pump light at a wavelength of about 775 nm is used toexcite ions into the ³ F₄ level. As mentioned above, a proportion ofthese excited ions decay into the ³ H₅ level, from where excitation tothe ¹ G₄ level is possible by absorption of pump light. In order toincrease the probability of this second excitation of pump energy, theapplicants have found that there should be several terbium ions close toevery thulium ion, so that cross-relaxation of population from the ³ H₄manifold in thulium over to the ⁷ F₀ manifold in terbium is enhanced,thus preventing population build up in the ³ H₄ manifold. However, asecond cross-relaxation mechanism also operates, this arising as resultof the of a close energy match between the 2.26 μm ground stateabsorption transition (⁷ F₆ -⁷ F₃) in terbium and the 2.31 μm emissiontransition (³ F₄ -³ H₅) in thulium (see FIG. 1). Due to thiscoincidence, the lifetime of the ³ F₄ manifold is reduced from around 15ms to around 0.5 ms, that is to say about two thirds of the populationexcited to the ³ F₄ manifold now decays non-radiatively into the ³ H₅manifold. Thus there is much greater probability that a second pumpphoton will be excited up to the ¹ G₄ manifold, leading to much largeramounts of spontaneous blue emission. FIG. 2 shows a plot of thevariation in observed blue fluorescence intensity with wavelength.

Referring to FIG. 3, a first form of laser according to the presentinvention is based on a length of standard formulation ZBLANfluorozirconate fibre 1 co-doped with thulium (0.1%) and terbium (1%)ions (ZBLAN denotes fluorides of Zr, Ba, La, Al and Na). The fibre 1 hasa numerical aperture of 0.205, a mode cut-off of about 2.0 μm, and acore diameter of 7.5 μm. The fibre background loss at both pump andsignal wavelengths is estimated to be around 1.2 dB/m. A simpleFabry-Perot laser cavity is formed by butting an input end 2 of thefibre 1 against a dielectric mirror 3, and by butting an output end 4 ofthe fibre against a dielectric mirror 5, the mirrors being highlyreflecting (HR≧99.5%) at between 460 nm and 490 nm, and highlytransmitting (HT≧80%) at between 760 nm and 800 nm. 775 nm pump light isderived from a laser 6 which is end-fire launched through the inputmirror 3. A pump blocking filter 7 is used to separate the remnant pumpand blue emission after transmission through the output mirror 5. Thelevel of reflectivity of the output mirror 5 can be reduced so thatgreater amounts of power are available for use.

FIG. 4 shows a second form of laser according to the invention. Thislaser is tunable, and is a modified version of the laser of FIG. 3.Accordingly, like reference numerals will be used for like parts, andonly the modifications will be described in detail. Thus, the cavity ofthe laser of FIG. 4 is provided with a cover slide 8 to preventreflections, a microscope objective 9 to collimate the laser output, anda three-plate birefrigent filter 10 to provide wavelength selectionwithin the cavity.

We claim:
 1. An optical amplifier comprising a fluorozirconate waveguideco-doped with thulium and terbium ions, and an optical pump meanscoupled to the waveguide for providing an optical pump signal capable ofexciting the thulium ions into the ¹ G₄ energy level, whereby theamplifier provides optical gain at about 475 nm, said optical pump meansbeing adapted to provide an optical pump signal having a wavelength inthe range of 770 nm to 790 nm.
 2. An amplifier as claimed in claim 1,wherein the optical pump signal has a wavelength of about 775 nm.
 3. Anamplifier as claimed in claim 1, wherein the waveguide is in the form ofa fibre.
 4. An amplifier as claimed in claim 1, wherein thefluorozirconate waveguide is doped with thulium ions to a concentrationof about 0.1%, and with terbium ions to a concentration of about 1%. 5.An amplifier as claimed in claim 1, wherein the optical pump means is asemiconductor laser diode.
 6. A laser comprising an optical amplifierand a pair of reflectors, the optical amplifier being as claimed inclaim 1, and the reflectors being positioned one at each end of thewaveguide, the reflectors defining a resonant cavity and havingreflectivities such as to provide reflections sufficient to sustainlasing action only at about 475 nm when the waveguide fibre is pumped bythe pumping means.
 7. A laser as claimed in claim 6, wherein thereflectors are mirrors.
 8. A method of amplifying optical signals whichmethod comprises providing signal radiation at 475 nm and pump radiationhaving a wavelength within the range of 770 nm to 790 nm into afluorozirconate waveguide co-doped with thulium and terbium ions wherebysaid pump radiation produces a population inversion in the thulium ions,said population inversion supporting amplification of said signalradiation.
 9. A method as claimed claim 8, wherein the thulium ions areexcited from the ground state (³ H₆) into the ¹ G₄ level by sequentialabsorption of two pump photons, said absorption producing a populationinversion between said ¹ G₄ level and the ground date.
 10. A method asclaimed in claim 9, wherein the first pump photon is absorbed by athulium ion to excite that ion from the ground state (³ H₆) to the level³ F₄, said excited ion decays to the ³ H₅ level by energy transfer tothe terbium ions, and the second pump photon is absorbed by a thuliumion that has decayed to the ³ H₅ level to excite that ion to the ¹ G₄level.
 11. An optical amplifier comprising a fluorozirconate waveguideco-doped with thulium and terbium ions, and an optical pump meanscoupled to the waveguide for providing an optical pump signal capable ofexciting the thulium ions into the ¹ G₄ energy level, whereby theamplifier provides optical gain at about 475 nm, said optical pump meansprovides an optical pump signal having a wavelength in the range of 770nm to 790 nm, and wherein the waveguide is doped with terbium ions to aconcentration which is several times the concentration of the thuliumions.
 12. A method as claimed in claim 8, wherein the pump radiation hasa wavelength of about 775 nm.
 13. A method of generating signalradiation at about 475 nm which method comprises providing pumpradiation having a wavelength within the range 770 nm to 790 nm into afluorozirconate waveguide co-doped with thulium and terbium ions wherebysaid pump radiation produces a population inversion in the thulium ionssuch that signal radiation at about 475 nm is produced wherein saidmethod comprises selectively returning said signal radiation into thefluorozirconate waveguide so as to sustain lasing action.
 14. A methodaccording to claim 13 wherein the pump radiation has a wavelength ofabout 775 nm.
 15. An optical amplifier comprising an input port foraccepting attenuated signals at 475 nm and an output port for providingsaid signals after amplification, said amplifier also comprising afluorozirconate waveguide co-doped with thulium and terbium ions, saidwaveguide interconnecting said input port and said output port, whereinsaid amplifier also comprises a pump coupled to the waveguide forproviding optical pump radiation having a wavelength in the range 770 nmto 790 nm wherein said pump radiation is adapted to excite the thuliumions to produce a population inversion which is capable of amplifyingthe signal received at the input port.